A transponder is actuated by electromagnetic energy in a radio signal generated by an interrogator and transmits various data to the interrogator, for example, as disclosed in Japanese unexamined patent publication JP-A 5-281347(1993). With a transponder incorporated in an automobile key, the key can be automatically identified, which is useful in preventing the automobile from being stolen. FIG. 10 shows a fundamental configuration associated with such a transponder. In an interrogator, an antenna coil 1 and a resonant capacitor 2 are connected in series, and a carrier signal source 4 is connected thereto through a switch 3. When the switch 3 is closed, a magnetic field 5 is generated at the antenna coil 1 in accordance with a signal from the carrier signal source 4 to excite an antenna coil 7 of a transponder 6. The transponder 6 accumulates energy from the magnetic field 5 as electric power by a receiving circuit 8, and transmits a predetermined certain identification code and the like from the antenna coil 7 to the antenna coil 1 using the accumulated electric energy.
In upper and lower halves of FIG. 11, there are respectively shown an electrical configuration of a driving circuit which drives the antenna coil 1 to supply electric power to the transponder 6, and signal waveforms during control over the magnetically coupled transponder as shown in FIG. 10. The switch 3 consists of an NPN transistor 9 and a PNP transistor 10. When a carrier signal from the carrier signal source 4 is supplied as a rectangular wave having an oscillation waveform at a frequency which is close to the resonant frequency of the antenna coil 1 and the resonant capacitor 2, the transistor 9 and the transistor 10 alternately conduct to provide a waveform which resembles a sine wave at a point S. The supply voltage Vcc is on the order of 5 V, and a current of, for example, 200 mA flows to the series resonant circuit of the antenna coil 1 and the resonant capacitor 2. A magnetically coupled transponder is provided with a halt period during which the carrier signal is turned off and a signal from the transponder is received or data are transmitted to the transponder by turning the carrier signal on and off.
According to the prior art as shown in FIG. 11, the waveform at the point S takes a long time .DELTA.t to attenuate even when the carrier signal having a rectangular waveform is turned off. When there is such a long attenuation time .DELTA.t during AM modulation by turning the carrier signal on and off, a long time is spent before a complete halt occurs even when the carrier signal is turned off, which disallows data communication at a high speed.
It is an object of the invention to provide an antenna driving device for a transponder, capable of rapidly attenuating an output during periods when the carrier signal is halted.